1. Field
The embodiments relate to wireless local area networks or WLANs, and more particularly WLANs capable of operating in accordance with IEEE802.11 standards.
2. Description of the Related Art
Since the introduction of the first IEEE802.11 standard in 1997, WLANs have become ubiquitous in homes, offices, airports and other public places as a way for wireless devices to access the Internet or a company Intranet. Although the IEEE802.11 group of standards is not the only standard applicable to WLANs, it is the pre-eminent standard and therefore, in the remainder of this specification, the term “WLAN” is to be understood as referring primarily to IEE802.11, without excluding other standards defining wireless networks operating on similar lines. IEEE802.11 is popularly known as Wi-Fi™. Wi-Fi is a trade mark of the Wi-Fi alliance, which defines Wi-Fi as “any wireless local area network (WLAN) products that are based on the Institute of Electrical and Electronic Engineers' (IEEE) 802.11 standards”.
There are essentially two types of WLAN topology: the ad-hoc network in which wireless devices communicate with each other without involving any central access points or any connection to a wired network, and, an infrastructure wireless network (of more relevance to the to be described) in which a so-called Access Point (AP), connected to a (wired) backhaul network such as broadband Internet, provides a bridge to a number of wireless devices in wireless communication with the AP. The AP is thus the nearest equivalent to a base station in a wireless cellular telephone system. The coverage area provided by one AP is referred to as a “hotspot”. Although each hotspot has typically a size of only tens of meters, a larger area can be covered by using multiple geographically-overlapping APs. The hotspots may thus be regarded as cells and the WLAN may form (or contribute to) a Small Cell Network (SCN). As in a cellular telephone system, handovers of mobile wireless devices from one AP to another are possible as the wireless devices move between APs.
The wireless devices (also called “clients” or “terminals”) may be laptop computers, tablets, smartphones or other portable devices, as well as peripheral devices such as printers, equipped with a radio capable of wireless communication on WLAN frequencies and in accordance with the applicable Wi-Fi standard. At least some of these client devices will be multi-RAT devices, capable of wireless communication in accordance with other radio access technologies (RATs) apart from Wi-Fi, in particular wireless cellular technologies such as 3G or LTE.
The AP may take the form of a wireless router or may be combined in another device, for example a Home eNB (HeNB) employed in an LTE wireless cellular system. Where several APs in a given area or building are connected to the same wired network, this is called a distribution system. An AP may also be provided by a suitably-equipped smart phone, which differs from conventional APs in that the connection to the backhaul network is wireless.
As Wi-Fi-capable devices proliferate, WLANs are becoming increasingly crowded; nevertheless WLAN may be the preferable option for wireless communication by multi-RAT devices in terms of coverage and/or data throughput, especially indoors where cellular wireless coverage may be patchy.
The IEEE802.11 standards relate to the data link layer and physical layer in the standard ISO seven-layer model, as depicted in FIG. 1.
The physical layer (PHY) depends on the specific standard (such as 802.11b, 802.11g, 802.11n and so on), and may also depend on the geographical area. Typically there are two operating frequency bands, 2.4 GHz and 5 GHz, each divided into a number of partially-overlapping channels, also called sub-channels (for example fourteen channels each 22 MHz wide). At any one time, an AP employs just one of these channels, on each of one or both frequency bands, for its data traffic with all the wireless devices connected to it. The AP also transmits beacon signals which the clients can scan for in order to detect APs within range.
Data is wirelessly transmitted in the form of packets each conforming to an Ethernet frame format, including source and destination IP addresses. An autorate algorithm is used to maximize the data rate over the wireless link.
Although only three of the available channels are completely non-overlapping, this allows for various APs to be deployed in a given geographical area without excessive mutual interference, by being set to different channels. On the other hand, using the same channel for all the wireless devices of an AP leads to the possibility of simultaneous access (collisions) by clients.
The Data Link Layer consists of two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC). The LLC is based on the same standards as other IEEE802 networks, allowing easy interfacing with the wired network.
The MAC layer of a WLAN handles access to the wireless medium by different clients, using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism. This is a “listen before talk” scheme used to avoid collisions by deferring access to the medium. For example, a device wishing to transmit data has to first listen to the channel for a predetermined amount of time to determine whether or not another device is transmitting on the channel within the wireless range. If the channel is sensed “idle”, then the device can start its transmission process. If the channel is sensed “busy”, the device defers its transmission for a random time period. To avoid collisions, client devices use a random backoff time after each frame to decide when to transmit, with the first transmitter seizing the channel. This is termed the contention-based medium access mechanism. The backoff time of each device generated is between 0 and the length of contention window (CW). The default size of the CW is set as part of contention configuration when the client associates with the AP. The request to send (RTS)/clear to send (CTS) procedure is used to further reduce the possibility of collisions for frames that are larger than the RTS threshold. FIG. 2 illustrates the CSMA/CA mechanism with RTS/CTS.
The contention-based medium access mechanism is adopted in both uplink (UL) and downlink (DL), and the AP is half duplex which means it can send or receive data but not both at the same time.
Another responsibility of the MAC layer is so-called association, which refers to connections of wireless devices to an AP. Each AP generally broadcasts an identifying code (SSID), or at least can be interrogated to provide it. Conventionally, when a wireless device detects SSIDs of APs within range, it chooses which AP it wishes to connect to—or “associate” with—based on signal strength or by user selection, and sends a request (Association/Reassociation Request) to the appropriate AP. If the AP accepts the request, it sends an Association Response and a wireless link is set up between the AP and wireless device (including any necessary authentication steps, depending on the security level of the WLAN). In this way a “session” is initiated for wireless communication between the wireless device and AP.
Thus, conventionally, the wireless device decides which AP to associate with, and this association is maintained for the duration of a session between the wireless device and AP, until the wireless link is terminated in some way, for example by the wireless device moving out of range of the AP. This causes a handover to another AP: the wireless device detects that a different AP would provide a better signal strength than the AP with which the wireless device is currently associated, so the wireless device “reassociates” with the new AP. The reassociation process is basically the same as for association, except that the client informs the new AP of the SSID of the previous AP. If the new AP uses a different channel from the previous AP then the wireless device will need to retune its radio accordingly.
FIG. 3 shows a state machine for a wireless device in a WLAN. There are three statuses defined for a wireless device, namely disconnected, connected and active. When clients enter a Wi-Fi network, with the typical equipment configurations, they select the APs to associate with based on the strongest received signal strength, as already mentioned. Clients after completing initial association and before starting any service sessions are regarded as being in the connected mode.
The dense deployment of small cell networks (SCN) has gained much attention in the mobile industry due to its potential for coping with fast growing data traffic. The almost ubiquitous broadband coverage and the popularity of smart phones and portable devices have led to the mobile data traffic surpassing voice on a global basis. It is predicted that there will be an exponential increase in mobile data traffic during the years to come. This development will be driven mainly by new, unique mobile applications (navigation, social networking, video telephony, etc.) being rich in multi-media content and increasing mobile broadband substitution through smart phones and USB-dongles. In reality, many operators have rolled out the dense deployment of SCN in the high traffic areas for coverage improvement and capacity boosting purposes. For example, Alcatel-Lucent and U.S. service provider Sprint announced an agreement on 6 Aug. 2012 to deploy Alcatel-Lucent's lightRadio™ Metro Cells to augment coverage in high-traffic areas; O2 launched free Wi-Fi services in central London before London 2012 Olympics, this Wi-Fi service being free to all users regardless of their cellular network provider.
The general term SCN covers a range of radio network design concepts which are all based on the idea of deploying BSs much smaller than the traditional macro cell devices to offer extended coverage (to indoor area or coverage hole areas of the existing macro cells) or support high demand in capacity. Small cells target at a coverage radius of 20-150 m and radiate at low power (0.01-10 W). Possible deployment scenarios include public indoor and outdoor with open or close access, residential and enterprise environments. In this sense, SCNs comprise low-power micro, pico and femto cells. These SCNs can be categorized as operator-managed SCNs.
In the contrast, the Wi-Fi APs deployed in a home or office using unlicensed spectrum can be regarded as forming a non operator-managed SCN. Nowadays in many home and office areas Wi-Fi access points have been widely used for a variety of services and applications. Since the Wi-Fi enabled smart phones and consumer electronics devices are increasingly entering the market, the utility of Wi-Fi will continue to grow. Both the operator-managed SCN and Wi-Fi SCN have their own advantages. It is very likely that 3G/4G femto cells and Wi-Fi access points will coexist in the future. In fact, there already are small cell access devices with 3G (and/or 4G) and Wi-Fi access technologies integrated in the same box coming out in the recent small cell market. Therefore the consumer and enterprise can greatly benefit from having both technologies available, for example, this can allow the user equipment (client) to optimally select the most appropriate wireless connection among different RATs or to simultaneously connect to multiple RATs and use the aggregated bandwidth from them. Furthermore, with the densification of small cell deployment in today's life, there is also a need to optimize the associations of clients with different APs using the same or different RATs. For example load balancing may be desirable to spread users and traffic more evenly among the available APs.
Attempts have been made to achieve load balancing among APs, but these generally involve causing an AP which is highly-loaded to refuse to accept further connections, which causes inconvenience to users.
OpenFlow™ is one technology of possible application to WLANs as well as to wired networks. Open Flow is a protocol which allows a server to provide rules to individual network switches on where to send packets, instead of each switch deciding for itself how to do so. Thus, in the context of a WLAN, Open Flow allows the possibility of centralized coordination of a number of commonly-connected APs, without however providing any template or approach for doing so. Open Flow is one example of a methodology called Software Defined Networking (SDN).
Currently with the typical configuration of clients that can support multimode RATs, they automatically select Wi-Fi APs for data traffic transmission once they are under the coverage of home or enterprise WLANs. Moreover clients typically select which AP to associate with using only locally available information and most of them use signal strength as the dominant factor in selecting an AP. However, this can result in some RATs and APs become more congested than the others which will cause the degradation in quality of experience (QoE) of clients. For example, when a lot of clients congregate in a conference room, they all tend to select the same Wi-Fi AP even when multiple APs operating on different channels are available. Therefore, in order to ease the traffic congestion and improve the QoE of client in this kind of scenarios, there is a need for optimization algorithms to manage the associations of clients with different APs using the same or different RATs.